Such parallel shaft transmissions are used in various powered machines including automobiles. In recent years, there has been a tendency that transmissions for use in vehicles are equipped with increasing numbers of speed change ratios because of demands for improvements in driving performance and of concerns on environmental effect. As a result, some transmissions that have more than five speed change ratios for forward drive have been developed for practical use. Generally, as the numbers of speed change ratios of transmissions increase, the numbers of gears disposed over the shafts also increase. This is said equally not only on vehicular transmissions but also on other types of transmissions. Therefore, transmissions tend to increase their sizes especially in the direction of their shafts. However, transmissions are designed to occupy certain limited amounts of space in specific machines or equipment in which they are intended to be installed. Therefore, various ideas have been applied to make the sizes of transmissions as compact as possible when the transmissions are designed to have increasing numbers of speed change ratios. Particularly in designing vehicular transmissions, whose sizes are limited by the sizes of respective vehicles accommodating the corresponding transmissions, it is important to take every measure that can make the transmissions as compact as possible.
Various such measures have been proposed to reduce the sizes of parallel shaft transmissions in axial direction. For example, a structural arrangement is known in which an extra shaft (intermediary shaft) is added between the input and output shafts of a transmission to reduce the number of gears per shaft (for example, refer to Japanese Laid-Open Patent Publication No. 2000-220700). In addition to this structural arrangement, another structural arrangement enables gears disposed on the output shaft to mesh with gears provided over the input shaft, to mesh also with gears provided over the intermediary shaft (for example, refer to Japanese Laid-Open Patent Publication No. 7(1995)-94854). In this arrangement, a gear on the output shaft is used commonly to establish two different speed change ratios, so that the number of gears disposed on the output shaft is made relatively small. Moreover, the gear on the input shaft and that on the intermediary shaft both meshing with such a commonly used gear on the output shaft are placed in a common plane in the transmission. As a result, the size of the transmission in the longitudinal direction of the shafts is much smaller than a similar transmission designed otherwise.
However, for recently developed transmissions with large numbers of speed change ratios, it is difficult to achieve a sufficient size reduction in axial direction by the above described structural arrangement in which a gear provided on the output shaft is used commonly for achieving two different speed change ratios. Therefore, there is a need for a new structural arrangement that can effect a size reduction in larger scale.